This invention relates to wireless communication systems and, more particularly, to wireless communication systems using multiple antennas at the transmitter and/or multiple antennas at the receiver.
The error free maximum information rate at which a communication system may communicate data, i.e., the capacity of the communication system, is governed by Shannon's formula. This maximum information rate is referred to as the Shannon limit. Wireless communication systems that use multiple antennas at the transmitter and optionally multiple antennas at the receiver, so-called multiple-input and/or multiple-output systems, can achieve dramatically improved capacity compared to single antenna systems, i.e., single antenna to single antenna systems. In random scattering propagation environments increasing the number of antennas at the receiver or at the transmitter (or both) produces a larger Shannon limit, i.e. a larger error free maximum information rate.
In multiple-input systems, a primitive data stream—the bits to be transmitted—is divided into a plurality of sub-streams, each of which is processed and transmitted. Each processed sub-stream can be transmitted over a respective different transmit antenna. It is often preferred, however, to cycle the processed sub-streams over all of the antennas so that successive segments of each processed sub-stream are transmitted over different ones of the transmit antennas cyclically.
The signals emanating from each transmit antenna arrive at each receive antenna. Thus, the received signal at each of the receive antennas (regardless of whether there are one or many receive antennas, the latter being a multiple-output system) is a superposition of each of the transmitted signals as modified by the channel characteristics. Even though the transmitted signals interfere with each other in each receive antenna signal, the received signals can be processed to separate the transmitted signals from one another and each separated-out signal can then be decoded to recover the respective sub-streams.
In particular, each of the transmitted signals needs to be received with a signal-to-noise ratio that is high enough to allow it to be sufficiently separated from the others so that its respective sub-stream can thereafter be decoded with an acceptable error rate. To this end, different combined-weighted-(remaining)-received-signals of receive antenna signals can be formed, the weights used to form any particular combination being such as to maximize the signal-to-noise ratio of a respective one of the transmitted signals. (See, for example, G. J. Foschini and M. J. Gans, “On Limits of Wireless Communications in a Fading Environment When Using Multiple Antennas,” Wireless Personal Communications, 6 Kluwer Academic Publishers, 1998, pp. 311–335, incorporated herein by this reference.) Even so, in order to get a high enough signal-to-noise ratio, the transmitted signals still have to be transmitted at either 1) a relatively low bit rate, or 2) some of the signals have to be transmitted at a relatively high power level. In the former case, this would reduce the overall bit rate of the primary data stream, and in the latter case, it would increase the overall power level of the primary data stream.
One advantageous method of implementing a multiple-input and/or multiple-output system that increases the primary data stream's bit rate without having to increase the power level, and thus increases the capacity of the system, involves staggering the start of the transmission of signals that represent the sub-streams. Thus, in a first time interval just one signal, representing just one sub-stream, is transmitted. In a second time interval this signal continues to be transmitted and the transmission of another signal, representing another sub-stream is begun, and so forth until all of the signals representing all of the sub-streams are being transmitted.
Thereafter, the receive antenna signals are processed to decode so-called layers of one of the signals that represents a respective one of the sub-streams. A layer is a portion of such a signal representing a particular sub-stream, such that the layer can be decoded with reference to only the symbols in the layer. The symbols representing a block code code-word, for example, can constitute a layer.
In particular, as described in more detail below, the receive antenna signals received over several time intervals are weighted with sets of weights to maximize the signal-to-noise ratio of one of the signals, representing a respective one of the sub-streams. The weighted receive antenna signals are combined and the resulting combined-weighted-received-signal is preprocessed to decode one layer of the signals. The decoded layer is re-encoded, reconstructed in the form that it was received by the receive antennas, and subtracted from the receive antenna signal, to produce the remaining receive antenna signals. The remaining receive antenna signals are then weighted with sets of weights to maximize the signal-to-noise ratio of another of the signals, representing another one of the sub-streams. These weighted remaining receive antenna signals are combined. The result is then preprocessed to decode one layer of the other signal. The decoded layer is re-encoded, reconstructed in the form that it was received by the receive antennas, and subtracted from the remaining receive antenna signal, and so on.
The just described decoding process takes advantage of the fact that during the first time interval only the signal representing one sub-stream is transmitted. Thus, there is no interference from signals representing the other sub-streams. As a result, during the first time interval the only transmitted signal has a relatively high signal-to-noise ratio. Assuming appropriate coding prior to transmission, this relatively high signal-to-noise ratio enables the first layer of this signal to be decoded with an acceptable error rate notwithstanding the presence in later time intervals of interference caused by layers of other signals. The decoded layer is then re-encoded and used to reconstruct the layer in the form that it was received by the receive antennas. The reconstructed first layer is then subtracted out of the receive antenna signals, eliminating it as source of interference for other layers.
The decoding of subsequent layers is carried out by subtracting from the receive antenna signals any already decoded and reconstructed layers, and treating the interference from the layers that have not been decoded as added noise. After a layer of a particular transmitted signal is decoded with an acceptable error rate it can then be re-encoded and subtracted from the remaining receive antenna signals—the receive antenna signals remaining after previously decoded layers have been subtracted from it. This process eliminates the decoded layer as source of interference for other layers. This process is repeated until all the layers of all of the signals are decoded. This approach allows for an increased bit rate because prior to separating out a particular layer the interference of other layers is removed for at least some of the particular layer. See, for example, U.S. Pat. No. 6,097,771 entitled “Wireless Communications System Having A Layered Space-Time Architecture Employing Multi-Element Antennas,” hereby incorporated by reference.